NateTut has asked for the wisdom of the Perl Monks concerning the following question:

I'm writing a generic user entry routine to capture strings from the user as well as an ending indicator. For example if they type a string and end with [Shift TAB] I will do one thing and if they end with [TAB] or [Enter] I will do something different. I am using the example "How do I read just one key without waiting for a return key?" from perlfaq8. So far so good. Well I quickly discovered that capturing certain keystrokes from a terminal isn't quite as easy as it is in a GUI environment. However I was able to figure out that [Shift TAB] gave me \e[Z. I also wanted to be able to trap for just [ESC] but I settled for [ESC q] instead. Now here's where the weirdness starts. I need to remove the ending character(s) from the string. With [Shift TAB] I was able to use: 
$DataRead{'StringRead'} = substr($DataRead{'StringRead'}, -3);

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but that didn't work with [ESC q]. Instead I ended up having to use: 
$DataRead{'StringRead'} = substr($DataRead{'StringRead'}, 0, length($D
+ataRead{'StringRead'}) - 2);

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This works fine, but leaves me wondering why the first approach didn't work. BTW Term::HotKey is just the module given in perlfaq8. I'm using perl v5.6.1 in on a Sun OS box using putty as an xterm. My test script follows: 
#!/usr/bin/perl
use strict;
use warnings;

use constant true             => 1;

use constant TAB              =>  9;
use constant LF               => 10;
use constant CR               => 13;
use constant ESC              => 27;
use constant BACKTAB          => -1;

$| = true;

use Term::HotKey;

#
# ReadFromKeyboard
#
# Read a key at Time from the Keyboard
#
# Arguments:
#
# Returns:     \%DataRead
#              'StringRead' => Characters Read
#               'ExitValue' => Character That Caused End of Read
#
# Thanks to Liverpole for the Original Version
#
sub ReadFromKeyboard
{
   my %DataRead;
   $DataRead{'StringRead'} = '';

   my $CharacterRead = '';

   while(true)
      {
      $CharacterRead = readkey();

      if($CharacterRead)
         {
         $DataRead{'StringRead'} .= $CharacterRead;
         print("\[$CharacterRead\]\[" . ord($CharacterRead) . "\]len\[
+" . length($CharacterRead) . "\] \[$DataRead{'StringRead'}\]\n");
         if($DataRead{'StringRead'} =~ /.*\t|\n|\r|\e\[Z|\eq$/)
            {
            if($DataRead{'StringRead'} =~ /.*\e\[Z$/)
               {
               $DataRead{'ExitValue'} = -1;
               $DataRead{'StringRead'} = substr($DataRead{'StringRead'
+}, -3);
               }
            elsif($DataRead{'StringRead'} =~ /.*\eq$/)
               {
               $DataRead{'ExitValue'} = 27;
               $DataRead{'StringRead'} = substr($DataRead{'StringRead'
+}, 0, length($DataRead{'StringRead'}) - 2);
               }
            else
               {
               $DataRead{'ExitValue'} = ord(chop($DataRead{'StringRead
+'}));
               }
            last();
            }
         }
      }
   return(\%DataRead);
}

# Main program
my $DataRead;
$DataRead->{'StringRead'} = '';
while(uc($DataRead->{'StringRead'}) ne 'QUIT')
   {
   $DataRead = ReadFromKeyboard();
   my $WhatIsIt;
   if($DataRead->{'ExitValue'} == TAB)
      {
      $WhatIsIt = 'TAB';
      }
   elsif($DataRead->{'ExitValue'} == LF)
      {
      $WhatIsIt = 'LF';
      }
   elsif($DataRead->{'ExitValue'} == CR)
      {
      $WhatIsIt = 'CR';
      }
   elsif($DataRead->{'ExitValue'} == ESC)
      {
      $WhatIsIt = 'ESC';
      }
   elsif($DataRead->{'ExitValue'} == BACKTAB)
      {
      $WhatIsIt = 'BACKTAB';
      }
   else
      {
      $WhatIsIt = 'IDK';
      }
   print("You Entered: \[$DataRead->{'StringRead'}\]\[" . length($Data
+Read->{'StringRead'}) . "\]\nYou Ended with a \[$WhatIsIt\]\[$DataRea
+d->{'ExitValue'}\]\n");
   }
__END__

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Replies are listed 'Best First'.
Re: Strange substr behavior
by gone2015 (Deacon) on Nov 20, 2008 at 18:39 UTC

    AFAIK, this:

      $DataRead{'StringRead'} = substr($DataRead{'StringRead'}, -3);

    [download]
    will set $DataRead{'StringRead'} to the last 3 characters of itself (or all if < 3 characters)... This:
      substr($DataRead{'StringRead'}, -3) = '' ;

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    would remove the last three characters (or all if < 3 characters). Shirley ?

    But in any case, would it not be easier to:

      if ($DataRead{'StringRead'} =~ s/(\t|\n|\r|\e\[Z|\eq)\z//) {
    my $term = $1 ;
    ....
  } ;

    [download]

[reply]
[d/l]
[select]
      Thanks! I realize now that I forgot to include an offset for substr. I'm not sure how it ever worked. Also I like your suggestion.
[reply]
[d/l]

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